1. Field of The Invention
The present invention relates generally to a receiving and transmitting arrangement. Particularly, the present invention relates to a method of transmitting electrical energy collected from a solar energy source to a remote target location from which a pilot signal is received.
2. Description of The Prior Art
Solar Power Satellites (SPS) have recently been proposed for collecting solar electrical energy and transmitting same to be received and utilized at remote locations. The collected energy would be transmitted as a microwave signal to, for example, an orbital space station, factory, or a location on earth or another celestial body. For establishing such as system of energy transfer, efficient receiving and transmission elements are required.
One such system of solar energy collection/transmission has been described in the Jul. 14, 1992 issue of the Asahi Newspaper, morning edition 13, page 15. FIG. 6 shows a representation of the described arrangement. Referring to the drawing, an earth launched solar energy collection/transmission satellite 101 is shown. The satellite is adapted to transmit solar energy in a direction from which a microwave pilot signal, aimed at the satellite from a remote location, is received.
For realizing such an energy transmission arrangement, for guiding an energy transmission wave and phase control of a generated microwave signal , a microwave pilot signal is emitted from a target point and the energy transmission arrangement must be active to transmit electrical energy back in a target direction from which the pilot signal is received. This has been attempted via phased array antennas and `retrodirective` transmission methods.
Referring to FIG. 7, such a retrodirective method as mentioned above will be explained. First, a pilot signal is emitted at a given frequency .omega..sub.i toward the position of the energy transmission arrangement (i.e. a satellite, not shown in the drawing), from a target point A. The pilot signal is received at a plurality of antenna elements (not shown) of the energy transmission arrangement. In response to reception of the pilot signal, the energy transmission arrangement emits an energy transmission wave at a given frequency .omega..sub.t, in the direction of the target point A. At a time t, when the energy transmission wave arrives at the target point A, a distance X.sub.0 is assumed to separate the target point A from a reference point P.sub.0 on the energy transmission arrangement. At this, a phase of the pilot signal in relation to the reference point P.sub.0 may be expressed as: EQU .phi..sub.0 =.omega..sub.i (t.sub.0 -X.sub.0 /C) (1)
wherein C=the speed of light.
In the same way, since the target point A separated from a different point, P.sub.1, on the energy transmission arrangement by an distance X.sub.1, a phase of the pilot signal may be expressed as: EQU .phi..sub.1 =.omega..sub.i (t.sub.0 -X.sub.1 /C) (2)
At this, a phase difference between the two points (P.sub.0, P.sub.1), may be expressed as: EQU .PHI..sub.i =.phi.1-.phi.0=-.omega..sub.i r/C (3)
wherein r=X.sub.1 -X.sub.0.
Provisionally, if the transmission wave is emitted at same phase from both points P.sub.0 and P.sub.1, a phase difference in relation to the target point A is present in the frequency .omega..sub.t of the transmission wave. Relating to the condition noted from equation (3) the phase difference of the transmission wave may be expressed as: EQU .PHI..sub.t =-.omega..sub.t r/C (4)
At this, while a phase of the transmission wave from the two points P.sub.0 and P.sub.1 are similar, a correction for the phase of the point P.sub.1 may be expressed as: EQU .PHI..sub.c =.omega..sub.t r/C (5)
According to this, phase correction for any number of emission points of the energy transmission arrangement may be effected according to the equation (5). Thus, the phase of emissions of the transmission wave from any point of the energy transmission arrangement can be converged at the target point A, the above being based on the general principles of the retrodirective method.
In applying the above transmission method to a solar energy transmission arrangement having a plurality of transmission antenna elements, a phase conjugation circuit is required for the received pilot signal for effecting retrodirective operation. According to this, an enormous quantity and weight may be accrued as the transmission antenna `subarray` is divided into a given number of blocks. Each block of the subarray must have its own pilot signal receiving apparatus and phase conjugation circuit. Also, each block of the subarray must have a voltage supply of the same phase for the transmission antenna elements. Since the microwave pilot signal emitted from the target is of substantially high voltage, and the frequency of the pilot signal is identical to the frequency of the energy transmission wave (also a microwave signal), it is difficult for the receiving apparatus of the energy transmission arrangement to extract the received pilot signal from the broadcast transmission wave. As applied to a solar energy transmission arrangement, for example, Mitsubishi Electronics proposes an asymmetric two frequency method in an article entitled `Microwave Transmission Antenna Beam Control Voltage` (1988).
Referring to FIG. 8, according to such an asymmetric two frequency method as set forth in the above-mentioned document, a phase conjugation circuit 110 is provided. This phase conjugation circuit 110 includes a wave divider device 111, which divides a pilot signal received by a solar energy transmission satellite (i.e. satellite 101 of FIG. 6) into a first phase a .theta..sub.1 (t) and a second phase .theta..sub.2 (t). The first phase signal .theta..sub.1 (t) is output from the wave divider device 111 to a first multiplier 112 while the second phase signal .theta..sub.2 (t) is output to a second multiplier 113. The first multiplier 112 multiplies the output of the wave divider 111 by -1, while the second multiplier 113 multiplies the output of the wave divider 111 by 2. After this, the outputs of the first and second multipliers 112 and 113 are input to a first mixer 114 where the -1 multiplied first phase signal .theta..sub.1 (t)and the 2 multiplied second phase signal .theta..sub.2 (t) are combined. The output of the mixer 114 is then input to a first band pass filter 115. From the mixer 114, via the band pass filter, either of a combined sum signal or difference signal are input to a second mixer 116. The second mixer 116 further receives a local signal 2.omega.tt of the same frequency as the pilot signal and, after combining this signal with the output of the first bandpass filter 116, outputs a combined signal to a second band pass filter 118. An output .omega.t(t) of the second bandpass filter 118 is then output from the phase conjugation circuit 110 to be sent to the antenna elements of the solar energy transmission satellite.
In the above-described solar energy transmission arrangement, beam control of the units of the subarray is adopted according to the retrodirective method. However, according to this arrangement, directional scanning within the microwave beam cannot be accomplished. Thus, if wide angle scanning of the microwave beam is to be accomplished, the size of the subarray must be limited to a relatively small size. Further, each subarray must be fitted with a conjugation circuit in order for the arrangement to function correctly.
For implementing such a conjugation circuit for the above asymmetric two frequency arrangement, circuit design and implementation becomes complex and costly as provision of such a asymmetric two frequency conjugation circuit for each antenna element is impractical. Thus it has been required to provide an electrical energy transmission system which may provide wide angle beam scanning with simple structure.